1. Field of the Invention
This invention generally relates to a block noise reducing apparatus. When a video signal is encoded into a compressed digital signal fit to be transmitted or recorded, block noise tends to occur in the compressed digital signal. In addition, when the compressed digital signal is decoded into the original video signal, block noise tends to occur in the decoding-resultant video signal. This invention specifically relates to an apparatus for reducing or removing such block noise in a video signal.
2. Description of the Related Art
In a known compressively encoding procedure, every frame represented by an input video signal is divided into rectangular blocks each having a plurality of pixels neighboring in horizontal and vertical directions, and the input video signal is compressed and encoded rectangular-block by rectangular-block to get an output video signal. The output video signal is transmitted via a recording medium or a transmission line before being decoded rectangular-block by rectangular-block to recover the original video signal.
In the case where the amount of information represented by an output video signal is required to be small, the rate of the compression of an input video signal is set to a large value. A difference in tone represented by the output video signal between adjacent rectangular blocks tends to be greater as the compression rate is larger. For example, a difference in tone represented by the output video signal between adjacent rectangular blocks is noticeable when a picture represented by the input video signal has only a gradual tone variation. Such a tone difference is referred to as block noise. In some cases, the block noise is noticeable in a picture represented by the output video signal.
Japanese patent application publication number P2001-119695A discloses a block noise reducing apparatus including a filter circuit for correcting a signal level difference at the boundary between adjacent rectangular blocks to reduce or remove block noise.
Specifically, the block noise reducing apparatus in Japanese patent application P2001-119695A includes an isolated differential point extraction circuit, a filter circuit, a delay circuit, and an adder. The isolated differential point extraction circuit differentiates an input video signal to get a differentiated video signal, and then extracts every portion of the differentiated video signal which corresponds to an isolated differential point. The extracted signal portions compose an isolated differential point signal. In general, isolated differential points correspond to tone differences between adjacent rectangular blocks which mean block noise. The filter circuit subjects the isolated differential point signal to a prescribed filtering process to get a corrective signal for canceling tone differences between adjacent rectangular blocks. For every isolated differential point, the corrective signal has effective values relating only to a limited picture area composed of 4 successive pixels. The center of the limited picture area corresponds to the isolated differential point. The delay circuit defers the input video signal to get a delayed video signal. The adder combines the delayed video signal and the corrective signal, thereby correcting the delayed video signal in response to the corrective signal to reduce or remove tone differences between adjacent rectangular blocks. The adder outputs the correction-resultant video signal as a block-noise-reduced video signal or a block-noise-removed video signal.
In the block noise reducing apparatus of Japanese patent application P2001-119695A, the corrective signal has effective values relating only to a limited picture area composed of 4 successive pixels centered at an isolated differential point. Thus, video signal portions relating to pixels other than the 4 successive pixels are uncorrected. In some cases, uncorrected video signal portions cause insufficient reduction or removal of block noise.
The block noise reducing apparatus of Japanese patent application P2001-119695A tends to deteriorate high frequency components of a video signal.
In the case where an input video signal represents a picture originally or naturally having tone differences between adjacent rectangular blocks, the block noise reducing apparatus of Japanese patent application P2001-119695A causes a deteriorated picture represented by the correction-resultant video signal.